Laboratory Maual

COLLEGE OF ENGINEERING AND INFORMATION TECHNOLOGY

ELECTRICAL ENGINEERING DEPARTMENT

LABORATORY MAUAL

EE 205

ELECTRIC CIRCUITS LAB

2015-16

TABLE OF CONTENTS

NUMBER EXPERIMENT TITLE PAGE NO.1 Ohm’s law 12 Unloaded voltage divider 53 Electrical power and work 7

4

Establishing and displaying characteristics in ac technology

11

4A Characteristics of the sine-wave voltage 134B Active power with sine wave voltage 15

5Capacitor in the ac circuit 19

5A Charging and discharging process of a capacitor 215B Phase shift between current and voltage on the

capacitor25

5C Capacitive reactance of a capacitor 275D Reactive power of a capacitor 29

6Coil in the ac circuit 33

6A Switch-on and switch off process of a coil 356B Phase shift between and voltage on a coil 396C Inductive reactance of a coil 416D Reactive power of a coil 43

7Interconnecting resistor, capacitor and coil 47

7A Series circuiting of resistor and capacitor 497B Parallel circuiting of resistor and capacitor 53

8 8A Series circuiting of a resistor and coil 578B Parallel circuiting of resistor and coil 61

9 Active, reactive and apparent power 65

10Transformer 69

10A Coupling factor 7110B Transformation ratio 73

Appendix 75

PREFACE

The EE 205 , Electric Circuits Lab is intended to teach the basics of Electrical Engineering to undergraduates of other engineering departments. The main aim is to provide hands-on experience to the students so that they are able to put theoretical concepts to practice.

The manual starts off with the basic laws such as Ohm's Law and Kirchhoff's Current and Voltage Laws. The important theorems of Thevenin and Norton are also provided along with the frequency domain analysis of circuits. They greatly simplify the complex electrical networks for analysis purposes. At the end, the students should be able to grasp the concepts thoroughly the electric circuits and able to apply them further in their field of study.

Experiment 1

Objective

The objective of this exercise is to become familiar with the measurement of resistance values using a digital multimeter (DMM). A second objective is to learn the resistor color code. Theory Overview The resistor is perhaps the most fundamental of all electrical devices. Its fundamental attribute is the restriction of electrical current flow: The greater the resistance, the greater the restriction of current. Resistance is measured in Ohms. The measurement of resistance in unpowered circuits may be performed with a digital multimeter. Like all components, resistors cannot be manufactured to perfection. That is, there will always be some variance of the true value of the component when compared to its nameplate or nominal value. For precision resistors, typically 1% tolerance or better, the nominal value is usually printed directly on the component. Normally, general purpose components, i.e. those worse than 1%, usually use a color code to indicate their value. The resistor color code typically uses 4 color bands. The first two bands indicate the precision values (i.e. the mantissa) while the third band indicates the power of ten applied (i.e. the number of zeroes to add). The fourth band indicates the tolerance. It is possible to find resistors with five or six bands but they will not be examined in this exercise. Examples are shown below: It is important to note that the physical size of the resistor indicates its power dissipation rating, not its ohmic value. Each color in the code represents a numeral. It starts with black and finishes with white, going through the rainbow in between:

0 Black 1 Brown 2 Red 3 Orange 4 Yellow 5 Green 6 Blue 7 Violet 8 Gray 9 White For the fourth, or tolerance, band: 5% Gold 10% Silver 20% None For example, a resistor with the color code brown-red-orange-silver would correspond to 1 2 followed by 3 zeroes, or 12,000 Ohms (more conveniently, 12 k Ohms). It would have a tolerance of 10% of 12 k Ohms or 1200 Ohms. This means that the actual value of any particular resistor with this code could be anywhere between 12,000-1200=10,800, to 12,000+1200=13,200. That is, 10.8 k to 13.2 k Ohms.

Resistor Color Code

Measurement of resistors with a DMM is a very straight forward process. Simply set the DMM to the resistance function and choose the first scale that is higher than the expected value. Clip the leads to the resistor and record the resulting value. Equipment (1) Digital Multimeter model:________________ srn:__________________

Procedure 1. Given the nominal values and tolerances in Table 3.1, determine and record the corresponding color code bands. 2. Given the color codes in Table 3.2, determine and record the nominal value, tolerance and the minimum and maximum acceptable values. 3. Obtain a resistor equal to the first value listed in Table 3.3. Determine the minimum and maximum acceptable values based on the nominal value and tolerance. Record these values in Table 3.3. Using the DMM, measured the actual value of the resistor and records it in Table 3.3. Determine the deviation percentage of this component and record it in Table 3.3. The deviation percentage may be found via: Deviation = 100 * (measured-nominal)/nominal. Circle the deviation if the resistor is out of tolerance. 4. Repeat Step 3 for the remaining resistor in Table 3.3.

Data Tables

Table 3.1

Value Color Band 1 Color Band 2 Color Band 3 Color Band 4

Table 3.2

Colors Value Tolerance Maximum Minimum

Table 3.3

Value Maximum Minimum Measured Deviation

Experiment 2

Objective

This exercise examines Ohm’s law, one of the fundamental laws governing electrical circuits. It states that voltage is equal to the product of current time’s resistance. Theory Overview

Ohm’s law is commonly written as V = I * R. That is, for a given current, an increase in resistance will result in a greater voltage. Alternately, for a given voltage, an increase in resistance will produce a decrease in current. As this is a first order linear equation, plotting current versus voltage for a fixed resistance will yield a straight line. The slope of this line is the conductance, and conductance is the reciprocal of resistance. Therefore, for a high resistance, the plot line will appear closer to the horizontal while a lower resistance will produce a more vertical plot line. Equipment (1) Adjustable DC Power Supply model (2) Digital Multimeter (3) Resistors

Schematic

Procedure

Ohm’s Law

1. Build the circuit using the resistor. Set the DMM to measure DC current and insert it in-

line between the source and resistor. Set the source for zero volts. Measure and record the current in Table 2.1. Note that the theoretical current is 0 and any measured value other than 0 would produce an undefined percent deviation.

2. Setting V at 2 volts, determine the theoretical current based on Ohm’s law and record this in Table2.1. Measure the actual current, determine the deviation, and record these in Table 2.1. Note that Deviation = 100 * (measured – theory) / theory.

3. Repeat step 2 for the remaining source voltages in Table 2.1.

4. Remove the 4.2 Ω and replace it with the 5.5 Ω. Repeat steps 1 through 3 using Table 2.2. 5. Remove the 5.5 Ω and replace it with the 22 Ω. Repeat steps 1 through 3 using Table 2.3. 6. Using the measured currents from Tables 2.1, 2.2, and 2.3, create a plot of current versus Voltage. Plot all three curves on the same graph. Voltage is the horizontal axis and current is the vertical axis.

DATA TABLES

Table 2.1 (4.2 Ω )

V (Volts) I (Theory) I (Measured ) Deviation2510

Table 2.2 (5.5 Ω )


Table 2.3 (22 Ω )


EXPERIMENT 3

Objective

The focus of this exercise is an examination of basic parallel DC circuits with resistors. A key element is Kirchhoff’s Current Law which states that the sum of currents entering a node must equal the sum of the currents exiting that node. The current divider rule will also be investigated.

Theory Overview

A parallel circuit is defined by the fact that all components share two common nodes. The voltage is the same across all components and will equal the applied source voltage. The total supplied current may be found by dividing the voltage source by the equivalent parallel resistance. It may also be found by summing the currents in all of the branches. The current through any resistor branch may be found by dividing the source voltage by the resistor value. Consequently, the currents in a parallel circuit are inversely proportional to the associated resistances. An alternate technique to find a particular current is the current divider rule. For a two resistor circuit this states that the current through one resistor is equal to the total current times the ratio of the other resistor to the total resistance.

Equipment

(1) Adjustable DC Power Supply (1) Digital Multimeter (2) Resistors

Schematic

Parallel DC Circuit

Figure 1

Procedure

1. Using the circuit of Figure 1 with R1 = ….. k, R2 = ……. k, and E = ……. volts, determine the theoretical voltage at A,B and C and record it in Table 1. Construct the circuit. Set the DMM to read DC voltage in the circuit at point A. Record this voltage in Table 1. Repeat the at points B and C.

2. Using the current divider rule, determine the current across each of the two resistors and record the values in Table 2 under the Theory column. Note that the larger the resistor, the smaller the current should be. Place the DMM probes between A and B and measure its total current.Record this value in table 2. Also determine the deviation. Repeat this process for the current through two resistors.

DATA TABLES

Table 1

Table 2

EXPERIMENT 4

Objective

The focus of this exercise is an examination of basic series DC circuits with resistors. A key element is Kirchhoff’s Voltage Law which states that the sum of voltage rises around a loop must equal the sum of the voltage drops. The voltage divider rule will also be investigated.

Theory Overview

A series circuit is defined by a single loop in which all components are arranged in daisy-chain fashion. The current is the same at all points in the loop and may be found by dividing the total voltage source by the total resistance. The voltage drops across any resistor may then be found by multiplying that current by the resistor value. Consequently, the voltage drops in a series circuit are directly proportional to the resistance. An alternate technique to find the voltage is the voltage divider rule. This states that the voltage across any resistor (or combination of resistors) is equal to the total voltage source times the ratio of the resistance of interest to the total resistance.

Equipment


Schematic

Series DC Circuit

Figure 1

Procedure

1. Using the circuit of Figure 1 with R1 = ….. k, R2 = ……. k, R3 = …… k, and E = ……. volts, determine the theoretical current and record it in Table 1. Construct the circuit. Set the DMM to read DC current and insert it in the circuit at point A. Record this current in Table 1. Repeat the current measurements at points B and C.

2. Using the voltage divider rule, determine the voltage drops across each of the three resistors and record the values in Table 2 under the Theory column. Note that the larger the resistor, the greater the voltage should be. Place the DMM probes across R1 and measure its voltage. Also determine the deviation. Repeat this process for the remaining three resistors.

DATA TABLES

Table 1

Table 2

EXPERIMENT 5

Objective

This exercise will involve the analysis of basic series-parallel DC circuits with resistors. The use of simple series-only and parallel-only sub-circuits is examined as one technique to solve for desired Currents and voltages

Theory Overview

Simple series-parallel networks may be viewed as interconnected series and parallel sub-networks. Each of these sub-networks may be analyzed through basic series and parallel techniques such as the application of voltage divider and current divider rules along with Kirchhoff’s Voltage and Current Laws. It is important to identify the most simple series and parallel connections in order to jump to more complex interconnections.

Equipment (1) Adjustable DC Power Supply (1) Digital Multimeter (3) Resistors

Schematic

Series-Parallel DC Circuit

Figure 1 Figure 2

Procedure

1. Consider the circuit of Figure 1 with R1 = 1 k, R2 = 2.2 k, R3 = 4.7 k and E = 10 volts. R2 is in parallel with R3. This combination is in series with R1. Therefore, the R2, R3 pair may be treated as a single resistance to form a series loop with R1. Based on this observation, determine the theoretical voltages at points A, B, and C with respect to ground. Record these values in Table 1. Construct the circuit. Set the DMM to read DC voltage and apply it to the circuit from point A to ground. Record this voltage in Table 1. Repeat the measurements at points B and C, determine the deviations, and record the values in Table 1.

2. Applying KCL to the parallel sub-network, the current entering node B (i.e., the current through R1) should equal the sum of the currents flowing through R2 and R3. These currents may be determined through Ohm’s Law and/or the Current Divider Rule. Compute these currents and record them in Table 2. Using the DMM as an ammeter, measure these three currents and record the values along with deviations in Table 2.

3. Consider the circuit of Figure 2. R2, R3 and R4 create a series sub-network. This sub-network is in parallel with R1. By observation then, the voltages at nodes A, B and C should be identical as in any parallel circuit of similar construction. Due to the series connection, the same current flows through R2, R3 and R4. Further, the voltages across R2, R3 and R4 should sum up to the voltage at node C, as in any similarly constructed series network. Finally, via KCL, the current exiting the source must equal the sum of the currents entering R1 and R2.

4. Build the circuit of Figure 2 with R1 = 1 k, R2 = 2.2 k, R3 = 4.7 k, R4 = 6.8 k and E = 20 volts. Using the series and parallel relations noted in Step 3, calculate the voltages at points B, C, D and E. Measure these potentials with the DMM, determine the deviations, and record the values in Table 3.

5. Calculate the currents leaving the source and flowing through R1 and R2. Record these values in Table 4. Using the DMM as an ammeter, measure those same currents, compute the deviations, and record the results in Table 4.

EXPERIMENT 6

Objective

The objective of this exercise is to continue the exploration of basic series-parallel DC circuits. The basic ladder network and bridge are examined. A key element here is the concept of loading, that is, the effect that a sub-circuit may have on a neighboring sub-circuit.

Theory Overview

Ladder networks are comprised of a series of alternating series and parallel connections. Each section effectively loads the prior section, meaning that the voltage and current of the prior section may change considerably if the loading section is removed. One possible technique for the solution of ladder networks is a series of cascading voltage dividers. Current dividers may also be used. In contrast, bridge networks typically make use of four elements arranged in dual series and parallel configuration. These are often used in measurement systems with the voltage of interest derived from the difference of two series sub-circuit voltages. As in the simpler series-parallel networks; KVL, KCL, the current divider rule and the voltage divider rule may be used in combination to analyze the sub-circuits.

Equipment(1) Adjustable DC Power Supply (1) Digital Multimeter (3) Resistors

Schematic

LADDERS AND BRIDGES

Figure 1 Figure 2

Procedure

1. Consider the circuit of Figure 1. R5 and R6 form a simple series connection. Together, they are in parallel with R4. Therefore the voltage across R4 must be the same as the sum of the voltages across R5 and R6. Similarly, the current entering node C from R3 must equal the sum of the currents flowing through R4 and R5. This three resistor combination is in series with R3 in much the same manner than R6 is in series with R5. These four resistors are in parallel with R2, and finally, these five resistors are in series with R1. Note that to find the voltage at node B the voltage divider rule may be used, however, it is important to note that VDR cannot be used in terms of R1 versus R2. Instead, R1 reacts against the entire series-parallel combination of R2 through R6. Similarly, R3 reacts against the combination of R4, R5 and R6. That is to say R5 and R6 load R4, and R3 through R6 load R2. Because of this process note that VD must be less than VC, which must be less than VB, which must be less than VA. Thus the circuit may be viewed as a sequence of loaded voltage dividers.

2. Construct the circuit of Figure 1 using R1 = 1 k, R2 = 2.2 k, R3 = 3.3 k, R4 = 6.8 k, R5 = 10 k, R6 = 22 k, and E = 20 volts. Based on the observations of Step 1, determine the theoretical voltages at nodes A, B, C and D, and record them in Table 1. Measure the potentials with a DMM, compute the deviations and record the results in Table 1.

3. Based on the theoretical voltages found in Table 1, determine the currents through R1, R2, R4 and R6. Record these values in Table 2. Measure the currents with a DMM, compute the deviations and record the results in Table 2.

4. Consider the circuit of Figure 2. In this bridge network, the voltage of interest is V AB. This may be directly computed from VA - VB. Assemble the circuit using R1 = 1 k, R2 = 2.2 k, R3 = 10 k, R4 = 6.8 k and E = 10 volts. Determine the theoretical values for V A, VB and VAB and record them in Table 3. Note that the voltage divider rule is very effective here as the R1 R2 branch and the R3 R4 branch are in parallel and therefore both “see” the source voltage. .

5. Use the DMM to measure the potentials at A and B with respect to ground, the red lead going to the point of interest and the black lead going to ground. To measure the voltage from A to B, the red lead is connected to point A while the black is connected to point B. Record these potentials in Table 3. Determine the deviations and record these in Table 3.

Table 1

Table 2

Table 3

EXPERIMENT 7

Objective

The objective of this exercise is to investigate the application of the superposition theorem to multiple DC source circuits in terms of both voltage and current measurements. Power calculations will also be examined.

Theory Overview

The superposition theorem states that in a linear bilateral multi-source DC circuit, the current through or voltage across any particular element may be determined by considering the contribution of each source independently, with the remaining sources replaced with their internal resistance. The contributions are then summed, paying attention to polarities, to find the total value. Superposition cannot in general be applied to non-linear circuits or to non-linear functions such as power.

Equipment(1) Adjustable DC Power Supply (1) Digital Multimeter (3) Resistors

Schematic

SUPERPOSITION THEOREM

Figure 1 Figure 2

Procedure

Voltage Application

1. Consider the dual supply circuit of Figure 1 using E1 = 10 volts, E2 = 15 volts, R1 = 4.7 k, R2 = 6.8 k and R3 = 10 k. To find the voltage from node A to ground, superposition may be used. Each source is considered by itself. First consider source E1 by assuming that E2 is replaced with its internal resistance (a short). Determine the voltage at node A using standard series-parallel techniques and record it in Table 1. Make sure to indicate the polarity. Repeat the process using E2 while shorting E1. Finally, sum these two voltages and record in Table 1

2. To verify the superposition theorem, the process may be implemented directly by measuring the contributions. Build the circuit of Figure 1 with the values specified in step 1, however, replace E2 with a short. Do not simply place a shorting wire across source E2! This will overload the power supply.

3. Measure the voltage at node A and record in Table 1. Be sure to note the polarity

4. Remove the shorting wire and insert source E2. Also, replace source E1 with a short. Measure the voltage at node A and record in Table 1. Be sure to note the polarity

5. Remove the shorting wire and re-insert source E1. Both sources should now be in the circuit. Measure the voltage at node A and record in Table 1. Be sure to note the polarity. Determine and record the deviations between theory and experimental results.

Current Application

6. Consider the dual supply circuit of Figure 2 using E1 = 10 volts, E2 = 15 volts, R1 = 4.7 k, R2 = 6.8 k, R3 = 10 k, R4 = 22 k and R5 = 33 k. To find the current through R4 flowing from node A to B, superposition may be used. Each source is again treated independently with the remaining sources replaced with their internal resistances. Calculate the current through R4 first considering E1 and then considering E2. Sum these results and record the three values in Table 2.

7. Assemble the circuit of Figure 2 using the values specified. Replace source E2 with a short and measure the current through R4. Be sure to note the direction of flow and record the result in Table 2.

8. Replace the short with source E2 and swap source E1 with a short. Measure the current through R4. Be sure to note the direction of flow and record the result in Table 2.

9. Remove the shorting wire and re-insert source E1. Both sources should now be in the circuit. Measure the current through R4 and record in Table 2. Be sure to note the direction. Determine and record the deviations between theory and experimental results.

10. Power is not a linear function as it is proportional to the square of either voltage or current. Consequently, superposition should not yield an accurate result when applied directly to power. Based on the measured currents in Table 2, calculate the power in R4 using E1-only and E2-only and record the values in Table 3. Adding these two powers yields the power as predicted by superposition. Determine this value and record it in Table 3. The true power in R4 may be determined from the total measured current flowing through it. Using the experimental current measured when both E1 and E2 were active (Table 2), determine the power in R4 and record it in Table 3.

Table 1

Table 2

EXPERIMENT 8

Objective

The objective of this exercise is to examine the use of Thevenin’s Theorem to create simpler versions of DC circuits as an aide to analysis. Multiple methods of experimentally obtaining the Thevenin resistance will be explored

Theory Overview

Thevenin’s Theorem for DC circuits states that any two port linear network may be replaced by a single voltage source with an appropriate internal resistance. The Thevenin equivalent will produce the same load current and voltage as the original circuit to any load. Consequently, if many different loads or sub-circuits are under consideration, using a Thevenin equivalent may prove to be a quicker analysis route than “reinventing the wheel” each time.

Thevenin’s Theorem

The Thevenin voltage is found by determining the open circuit output voltage. The Thevenin resistance is found by replacing any DC sources with their internal resistances and determining the resulting combined resistance as seen from the two ports using standard series-parallel analysis techniques. In the laboratory, the Thevenin resistance may be found using an ohmmeter (again, replacing the sources with their internal resistances) or by using the matched load technique. The matched load technique involves replacing the load with a variable resistance and then adjusting it until the load voltage is precisely one half of the unloaded voltage. This would imply that the other half of the voltage must be dropped across the equivalent Thevenin resistance, and as the Thevenin circuit is a simple series loop then the two resistances must be equal as they have identical currents and voltages

Equipment


Schematic

Figure 1 Figure 2

Procedure

1. Consider the circuit of Figure 1 using E = 10 volts, R1 = 3.3 k, R2 = 6.8 k, R3 = 4.7 k and R4 (RLoad) = 8.2 k. This circuit may be analyzed using standard series-parallel techniques. Determine the voltage across the load, R4, and record it in Table 1. Repeat the process using 2.2 k for R4.

2. Build the circuit of Figure 1 using the values specified in step one, with R Load = 8.2 k. Measure the load voltage and record it in Table 1. Repeat this with a 2.2 k load resistance. Determine and record the deviations. Do not deconstruct the circuit.

3. Determine the theoretical Thevenin voltage of the circuit of Figure 1 by finding the open circuit output voltage. That is, replace the load with an open and calculate the voltage produced between the two open terminals. Record this voltage in Table 2.

4. To calculate the theoretical Thevenin resistance, first remove the load and then replace the source with its internal resistance (ideally, a short). Finally, determine the combination series-parallel resistance as seen from the where the load used to be. Record this resistance in Table 2.

5. The experimental Thevenin voltage maybe determined by measuring the open circuit output voltage. Simply remove the load from the circuit of step one and then replace it with a voltmeter. Record this value in Table 2.

6. There are two methods to measure the experimental Thevenin resistance. For the first method, using the circuit of step one, replace the source with a short. Then replace the load with the ohmmeter. The Thevenin resistance may now be measured directly. Record this value in Table 2.

7. In powered circuits, ohmmeters are not effective while power is applied. An alternate method relies on measuring the effect of the load resistance. Return the voltage source to the circuit, replacing the short from step six. For the load, insert either the decade box or the potentiometer. Adjust this device until the load voltage is half of the open circuit voltage measured in step five and record in Table 2. under “Method 2”. At this point, the load and the Thevenin resistance form a simple series loop as seen in Figure 2. This means that they “see” the same current. If the load exhibits one half of the Thevenin voltage then the other half must be dropped across the Thevenin resistance, that is VRL = VRTH. Consequently, the resistances have the same voltage and current, and therefore must have the same resistance according to Ohm’s Law. 8. Consider the Thevenin equivalent of Figure 2 using the theoretical ETH and RTH from Table 2 along with 8.2 k for the load (RL). Calculate the load voltage and record it in Table 11.3. Repeat the process for a 2.2 k load. . 9. Build the circuit of Figure 2 using the measured ETH and RTH from Table 2 along with 8.2 k for the load (RL). Measure the load voltage and record it in Table 3. Also determine and record the deviation.

10. Repeat step nine using a 2.2 k load.

Table 1

Table 2

Table 3

EXPERIMENT 9

Objective

The objective of this exercise is to determine the conditions under which a load will produce maximum power. Further, the variance of load power and system efficiency will be examined graphically.

Theory Overview

In order to achieve the maximum load power in a DC circuit, the load resistance must equal the driving resistance, that is, the internal resistance of the source. Any load resistance value above or below this will produce a smaller load power. System efficiency (η) is 50% at the maximum power case. This is because the load and the internal resistance form a basic series loop, and as they have the same value, they must exhibit equal currents and voltages, and hence equal powers.

Maximum Power Transfer

As the load increases in resistance beyond the maximizing value the load voltage will rise, however, the load current will drop by a greater amount yielding a lower load power. Although this is not the maximum load power, this will represent a larger percentage of total power produced, and thus a greater efficiency (the ratio of load power to total power).Equipment

(1) Adjustable DC Power Supply (1) Digital Multimeter (3) Resistors decade box(4) Resistor

Schematic

Figure 1

Procedure

1. Consider the simple the series circuit of Figure 1 using E = 10 volts and Ri = 3.3 k. Ri forms a simple voltage divider with RL. The power in the load is VL2/RL and the total circuit power is E2/(Ri+RL). The larger the value of RL, the greater the load voltage, however, this does not mean that very large values of RL will produce maximum load power due to the division by RL. That is, at some point VL2 will grow more slowly than RL itself. This crossover point should occur when RL is equal to Ri. Further, note that as RL increases, total circuit power decreases due to increasing total resistance. This should lead to an increase in efficiency. An alternate way of looking at the efficiency question is to note that as RL increases, circuit current decreases. As power is directly proportional to the square of current, as RL increases the power in Ri must decrease leaving a larger percentage of total power going to RL

2. Using RL = 30, compute the expected values for load voltage, load power, total power and efficiency, and record them in Table 1. Repeat for the remaining RL values in the Table. For the middle entry labeled Actual, insert the measured value of the 3.3 k used for Ri.

3. Build the circuit of Figure 1 using E = 10 volts and Ri = 3.3 k. Use the decade box for RL and set it to 30 Ohms. Measure the load voltage and record it in Table 2. Calculate the load power, total power and efficiency, and record these values in Table 2. Repeat for the remaining resistor values in the table

4. Create two plots of the load power versus the load resistance value using the data from the two tables, one for theoretical, one for experimental. For best results make sure that the horizontal axis (RL) uses a log scaling instead of linear.

5. Create two plots of the efficiency versus the load resistance value using the data from the two tables, one for theoretical, one for experimental. For best results make sure that the horizontal axis (RL) uses a log scaling instead of linear.

Table 1

Table 2

EXPERIMENT 10

Objective

The objective of this exercise is to examine the practical workings of adjustable resistances, namely the potentiometer and rheostat. Their usage in adjustable voltage and current control will be investigated.

Theory Overview

A potentiometer is a three terminal resistive device. The outer terminals present a constant resistance which is the nominal value of the device. A third terminal, called the wiper arm, is in essence a contact point that can be moved along the resistance. Thus, the resistance seen from one outer terminal to the wiper plus the resistance from the wiper to the other outer terminal will always equal the nominal resistance of the device. This three terminal configuration is used typically to adjust voltage via the voltage divider rule, hence the name potentiometer, or pot for short. While the resistance change is often linear with rotation (i.e., rotating the shaft 50% yields 50% resistance), other schemes, called tapers, are also found. One common non-linear taper is the logarithmic taper. It is important to note that linearity can be compromised (sometimes on

Potentiometers and Rheostats

purpose) if the resistance loading the potentiometer is not significantly larger in value than the potentiometer itself. If only a single outer terminal and the wiper are used, the device is merely an adjustable resistor and is referred to as a rheostat. These may be placed in-line with a load to control the load current, the greater the resistance, the smaller the current..Equipment

(1) Adjustable DC Power Supply (1) Digital Multimeter (3) Potentiometer(4) Resistor

Schematic

Figure 1 Figure 2 Figure 3

Procedure

1.A typical potentiometer is shown in Figure 1. Using a 10 k pot, first rotate the knob fully counter-clockwise and using the DMM, measure the resistance from terminal A to the wiper arm, W. Then measure the value from the wiper arm to terminal B. Record these values in Table 1. Add the two readings, placing the result in the final column.

2. Rotate the knob 1/4 turn clockwise and repeat the measurements of step 1. Repeat this process for the remaining knob positions in Table 1. Note that the results of the final column should all equal the nominal value of the potentiometer.

3. Construct the circuit of Figure 2 using E = 10 volts, a 10 k potentiometer and leave RL open. Rotate the knob fully counter-clockwise and measure the voltage from the wiper to ground. Record this value in Table 2. Continue taking and recording voltages as the knob is rotated to the other four positions in Table 2.

4. Set RL to 47 k and repeat step 3.

5. Set RL to 4.7 k and repeat step 3.

6. Set RL to 1 k and repeat step 3.

7. Using a linear grid, plot the voltages of Table 2 versus position. Note that there will be four curves created, one for each load, but place them on a single graph. Note how the variance of the load affects the linearity and control of the voltage.

8. Construct the circuit of Figure 3 using E = 10 volts, a 100 k potentiometer and RL = 1 k. Rotate the knob fully counter-clockwise and measure the current through the load. Record this value in Table 3. Repeat this process for the remaining knob positions in Table 3.

9. Replace the load resistor with a 4.7 k and repeat step 8.

Table 1

Table 2

Table 3

APPENDIX I

Resistor Color Code

APPENDIX II

LABORATORY REGULATIONS AND SAFETY RULES

The following Regulations and Safety Rules must be observed in all concerned laboratory location.

1. It is the duty of all concerned who use any electrical laboratory to take all reasonable steps to safeguard the HEALTH and SAFETY of themselves and all other users and visitors.

2. Be sure that all equipment are properly working before using them for laboratory exercises. Any defective equipment must be reported immediately to the Lab. Instructors or Lab. Technical Staff.

3. Students are allowed to use only the equipment provided in the experiment manual or equipment used for senior project laboratory.

4. Power supply terminals connected to any circuit are only energized with the presence of the Instructor or Lab. Staff.

5. Students should keep a safety distance from the circuit breakers, electric circuits or any moving parts during the experiment.

6. Avoid any part of your body to be connected to the energized circuit and ground.

7. Switch off the equipment and disconnect the power supplies from the circuit before leaving the laboratory.

8. Observe cleanliness and proper laboratory house keeping of the equipment and other related accessories.

9. Wear the proper clothes and safety gloves or goggles required in working areas that involves fabrications of printed circuit boards, chemicals process control system , antenna communication equipment and laser facility laboratories.

10. Double check your circuit connections specifically in handling electrical power machines, AC motors and generators before switching “ON” the power supply.

11. Make sure that the last connection to be made in your circuit is the power supply and first thing to be disconnected is also the power supply.

12. Equipment should not be removed, transferred to any location without permission from the laboratory staff.

13. Software installation in any computer laboratory are not allowed without the permission from the Laboratory Staff.

14. Computer games are strictly prohibited in the computer laboratory.

15. Students are not allowed to use any equipment without proper orientation and actual hands on equipment operation.

16. Smoking and drinking in the laboratory are not permitted.

All these rules and regulations are necessary precaution in Electrical Laboratory to safeguard the students, laboratory staff, the equipment and other laboratory users.

Documents

Laboratory Maual